In a small saucepan, add yolks and whisk very well until they get thick, about 1 minute. While doing that, pour in lemon juice and season with salt and pepper.
Move saucepan over medium heat and keep lifting or moving the saucepan off the heat, whisking constantly. The trick here is to not shock the yolks – gradually heat up the yolks.
When the custard gets thick, move the saucepan away from the heat and start adding the butter by pouring a few drops at first (don’t shock the yolks), whisking constantly. Gradually add more, continuing whisking, until the sauce is nice and smooth.
If not using the sauce right away, move it to the back burner, keeping it warm on very low temperature. Makes 1 cup.
Note: Depending on which recipe the sauce will be used, add other ingredients or season it differently.